Fluid is floating instead of falling?
The main reason for that behaviour are non-manifold mesh and unapplied transforms.


Non-manifold mesh used as obstacle.
One of the common options for different fluid objects is Volume Initialization. Only one value, which is Shell, can work regardless whether mesh is opened, i.e. non-manifold, or not. Two others assume that mesh is closed and thus won't initialize volume of obstacle (in this case) as expected.

In your file the sink object, where water is supposed to fall down after flooding the top box, is a plane with a Solidify modifier for adding thickness:


Non-manifold geometry is shown as selected

The object is set up to use Volume Initialization > Both which requires manifold object (you can change it to Shell though and simulation will work).  

The mentioned modifier, supposed to make the object as manifold, won't be taken into account as it is placed below Fluid one in the stack of modifiers. However you can't move it above Fluid modifier because the latter requires original data of the object to work. Thus you end up either with applying Solidify or extruding faces in Edit mode.  
Unapplied transforms on objects used in simulation

In order modifier to work as expected, transforms (scale, rotating) should be applied to object data. See more in Why is it important to apply transformation to an objects data?.
They aren't applied for all objects used in simulation. It is important at least for object with Solidify modifier for correct adding thickness to the mesh (and I recommend making its faces planar, e.g. with Mesh > Clean-up > Make Planar Faces). Note that leaving other transforms unapplied may lead to unexpected results too (although fluid simulation will work).

With that done, fluid will act as expected: